(1) Field of the Invention
The present invention involves the aircraft field, and pertains to procedures for measuring the speed of the aircraft in relation to a surrounding fluid. More specifically, the present invention relates to the procedures for determining the airspeed of a rotorcraft performing evolutions during stationary flight and/or at low speeds.
(2) Description of Related Art
The present invention relates to a procedure for the determination of the current airspeeds of a rotorcraft performing evolutions during stationary flight and/or at low speeds. The determined airspeeds correspond in particular to a longitudinal airspeed LASVy and a lateral airspeed LASVx of the rotorcraft. The concepts of stationary and low-speed flight should be viewed as being analogous to a rotorcraft flight regime evolving at a speed that is less than or equal to a forward-speed threshold of a rotorcraft as generally accepted. For illustrative purposes but non-restrictively, such a speed threshold corresponds to a forward speed of the rotorcraft that is, for example, lower than 40 knots (Kt).
Rotorcraft are aircraft that have the particular capability of being able to perform evolutions in stationary flight and/or at low speeds. Rotorcraft include at least one main rotor that provides their support and often also their propulsion as well, and at least one anti-torque device, such as, for example, an anti-torque rotor, it makes it possible to control the movements of the rotorcraft in yaw. These rotors include a rotating wing that includes blades. To modify the forward modes of the rotorcraft, a pilot causes a change in the pitch of the blades, which change is at least collective but may also be cyclic. The anti-torque rotor consists, for example, of a rotorcraft tail rotor, or, for hybrid helicopters, at least one propulsive propeller. The pilot may be an operator who manually controls various command units, and/or may be a device, such as in particular an autopilot, that provides piloting assistance.
To assist the pilot, rotorcraft are equipped with onboard instrumentation that provides various pieces of information about the forward modes and/or about the environment outside the rotorcraft.
In the field of aeronautics, the onboard instrumentation currently includes measurement instruments for determining the current airspeeds of aircraft, namely, the longitudinal airspeed LASVx and the lateral airspeed LASVy. The concepts of “longitudinal” and “lateral” are commonly accepted notions within the context of the overall orientation of the aircraft. The airspeed-measurement instruments consist, in particular, of anemometric probes located outside of the aircraft, which provide static-pressure and dynamic-pressure information, based on which an airspeed of the aircraft is determined.
One problem that arises for rotorcraft is the lack of reliability and pertinence of the information provided by the anemometric probes when the rotorcraft is moving forward at low speeds. Furthermore, the installation of probes on the outside of the rotorcraft affects its aerodynamics, which should be avoided. Moreover, the information provided by the probes depends, on the one hand, on the orientation of the probes on the rotorcraft, which makes their positioning difficult, and, on the other hand, on the orientation of their installation on the rotorcraft.
Nevertheless, it is useful and even necessary to provide to the pilot of the rotorcraft, in a stationary and/or low-speed flight regime, information about the airspeeds and about the direction of the rotorcraft in terms of its attitude in relation to the flow of the surrounding air. Such information is useful, in particular, to indicate the intensity and/or direction of the wind, and even to warn of in-flight hazards that might be induced by a backwind component.
To solve this problem, VIMI [“Vitesse Indiquée par Moyens Internes”] (Speed Indicated by Internal Means) or AVC [“Anémométrie Vectorielle Calculée”] (Calculated Vectorial Anemometry) calculation systems have been proposed that make it possible to determine the airspeeds of a rotorcraft based on simplified theoretical flight-mechanics formulas.
Such theoretical formulas take into consideration, in particular, various pieces of information provided by the onboard instrumentation, such as instruments that indicate the positions of flight controls and/or indicate the accelerations of the rotorcraft. For example, the calculation systems still uses information supplied by probes mounted on the axis of rotation of the main rotor, or mounted at the end of the blades of such a rotor.
For example, according to document FR 2885222 (THALES S. A.), a law for estimating the airspeeds of a rotorcraft is defined according to a combination of various pieces of measurement information supplied by the onboard instrumentation. Such information relates to a measurement of the pressure of the air outside the rotorcraft; the position of the kinematic chain employed to vary the pitch of the main rotor blades in order to determine the attitude of the rotorcraft; and an acceleration of the equipment, as measured on the basis of an onboard inertial measurement unit.
As a further example, according to document FR 2613078 (CROUZET), a pair of laws makes it possible to calculate the current airspeeds of the rotorcraft based on airspeeds measured in flight onboard a test rotorcraft, taking into consideration various calculation criteria relating to the pitch of the rotor blades of the rotorcraft, to the acceleration of the rotorcraft, and to the inclination of rotorcraft in pitch and roll.
As yet a further example, according to document EPO 402224 (AVIONIC SEXTANT), a calibration operation is performed in flight onboard a test rotorcraft to estimate the airspeeds of a rotorcraft in accordance with various criteria. Then, laws for the calculation of such airspeeds of any rotorcraft that is analogous to the test rotorcraft are constructed on the basis of the results of the calibration operation.
The VIMI calculation systems require a calibration of the simplified theoretical formulas, which calibration consists of a modest adaptation of these simplified theoretical formulas. Such a calibration is performed for each of the rotorcraft on which the systems are present, so as to obtain satisfactory accuracy and reliability with regard to the determined airspeeds. The law governing the estimation of the airspeeds, as incorporated into the calculation system, is calibrated during test flights of the rotorcraft. More specifically, the pertinence and reliability of the application of the estimation law depend on the structural characteristics of the rotorcraft instruments that are used to apply the estimation law.
Consequently, in order to take into account the dependence, the parameters of the estimation law must be calibrated on the basis of test flights performed for each of the rotorcraft onboard which the calculation system is specifically present. Such calibration operations are cumbersome, lengthy, and expensive.